Trucks

LNG and CNG look to be a medium-duty hit – but will they work in Class 8?

By Jack Roberts

Dumps and other vocational trucks that return to a central fueling facility each day make ideal candidates for CNG or LNG consideration.

Natural gas as a fuel for internal combustion engines isn’t a new idea. What is new is the scope of its penetration into new vehicle applications in the wake of rising diesel prices – and nowhere is that trend more prominent than the fuel’s current inroads into the heavy-duty truck market.

Fleet interest in natural gas as a viable Class 8 fuel skyrocketed during the unprecedented 2007-08 fuel price spike, but that brief affair ended as prices returned to more manageable levels in recent years.

Today, interest is returning because fleets realize that yesterday’s lower fuel prices truly are a thing of the past, says Robert Carrick, vocational sales manager of natural gas for Freightliner. “Every time the prices retreat, they’ll stay at a higher level instead of dropping all the way back down,” Carrick says. “Fleets are looking at ways to combat that.”

Even if the natural gas infrastructure is lagging, competition is driving the heavy-duty truck marketplace, says Jonathan Burke, vice president of global market development for Westport.

“The challenge for fleets that looked to natural gas in the past was that there were very few vehicle models and engines available,” Burke says. “Now we’re seeing all of the major truck OEMs step up and invest significant amounts of capital in making a huge range of new factory-built vehicles available today. That’s the sea change.”

The right path

Both compressed natural gas (CNG) and liquefied natural gas (LNG) fuels behave exactly the same once introduced into an engine’s combustion chamber, but the manner in which they are delivered, stored, packaged on a vehicle and transported to the combustion chamber are radically different.

“Fleets operating on longer routes should ensure they are running on a defined natural gas corridor,” says Steve Weiner, alternative fuels marketing manager for Peterbilt. “If the routes are stable, then you can put fuel where you need it. If the routes change frequently, then it can be challenging to ensure fuel availability.”

Natural gas costs less than diesel, but you need more of it to get the same range, hence the large fuel storage tanks behind the cab.

Fleet managers can follow a simple rule of thumb when looking to convert from diesel to CNG or LNG, says Nadine Haupt, director of alternative fuels for Navistar. Take a gallon of diesel fuel, and consider the amount of energy that it creates; to get that same amount of energy using LNG, about twice as much fuel is needed. The ratio for CNG is about four times as much as diesel.

On smaller routes, fleets tend to spec CNG because the tank packaging and weight considerations on the chassis are more doable. But anybody on longhaul, or even regional haul, might look at LNG depending on weight and packaging. “Then you have to take a look at what’s available in terms of infrastructure,” Haupt says.

CNG fuel systems store compressed gas in cylindrical tanks at pressures up to 3,600 psi. CNG requires compression to get the fuel into the tanks, and stations are designed for refueling vehicles to meet fleet requirements ranging from a few minutes (fast fill) to several hours (slow fill). Fast fill can increase the temperature of the gas, which can result in a lower fill volume. LNG, stored as a liquid, is pumped into vehicle tanks in a similar fashion as diesel or gasoline, but since LNG is a cryogenic (cold) fuel, protective clothing and eyewear should be worn.

A look at refuse and many medium-duty applications showcases the differences between CNG and LNG, says Steve Gilligan, vice president of product and marketing for Navistar. “These are trucks that typically come home every night to the same location, and they’re generally fueled overnight,” Gilligan says. “That scenario lends itself perfectly to CNG. The trucks can be hooked up to a slow fill, fill overnight and the next day are ready to go.”

Running Class 8 trucks over the road is a different scenario completely, Gilligan says. If the trucks are not coming home every night, they need enough range to get to the next available fuel station. “If you’re running longer distances or need a more compact vehicle, you need to look at LNG because you get a better packaging on the vehicle and faster refueling times,” he says.

But the CNG-LNG debate is not like the old VHS-Beta dispute back in the 1970s. “There’s not going to be one fuel that emerges as ‘The One,’ ” Gilligan says. “Both have their place in fleet operations. It’s really more like choosing between gasoline or diesel. You just have to determine which path is right for you.”

Engine options

New engines are on rapid development tracks throughout the industry, but pull open the hood on a natural gas truck today, and odds are there’s a red Cummins block sitting there. Cummins, through its Cummins-Westport joint venture, currently is the largest provider of natural-gas heavy-duty engines in North America with 8.9-liter and soon-to-be-released Cummins Westport 12- and proprietary 15-liter offerings based on its ISX diesel engine.

There are three versions of heavy-duty natural gas engines currently in development or available for purchase in North America – regardless of whether CNG or LNG is spec’d on the vehicle. Cummins Westport employs a spark-ignited engine, which works in a conventional manner: A fuel-air mixture entering the cylinder is ignited by a spark plug, providing lower compression ratios.

Navistar is working on the next engine technology, Clean Air Power, a “dual-fuel” system in which diesel fuel and natural gas are fumigated in the compression chamber and mixed with diesel fuel injected into the cylinder on a displacement ratio to ignite the mixture. While the power curve is close to that of conventional all-diesel powertrains, there are weight and packaging issues since large amounts of two different fuel types have to be carried on the vehicle. Still, this is an attractive option for fleets that have high-horsepower and torque requirements but want to cut diesel fuel costs.

The third technology is Westport’s high-pressure direct-injection (HPDI) engine, which is offered in a big-bore 15-liter configuration based on the Cummins ISX diesel engine platform. The pure diesel combustion cycle uses a fuel injector in the cylinder; the engine looks exactly the same as a conventional diesel, but the injector sends diesel through one nozzle and natural gas through another.

At the end of the compression stroke, the engine is compressing 100 percent air with the same compression ratio as a conventional diesel; at that point, a miniscule amount of diesel is injected to initiate combustion. Next, through the same injector but a different nozzle, the bulk shot of natural gas enters the chamber and is ignited in a process that uses about 95 percent natural gas and 5 percent diesel.

In an on-road situation, that process could use even less diesel because the diesel injection is consistent; it goes in at the same rate all the time, so when under full power on the highway, the truck is using predominantly natural gas.

Notes from the real world

Despite its limitations in terms of power density and range when compared to diesel, natural gas appears to be working for fleets willing to accept those tradeoffs in return for lower fuel prices, a greener operation and a dynamic push to wean the United States off foreign oil.

Paper Transport is running two natural gas Freightliners with Cummins-Westport ISL G engines from its home base in Green Bay, Wis., to Chicago twice a day and says everything is working well. “We started looking at this in August of 2009 and were up and running in February of 2010,” says Jeff Shefchik, president of the 320-truck fleet.

A sticking point for many fleet managers is the significantly higher acquisition costs of natural gas engines – a problem that Carrick says is unlikely to go away as economies of scale come into play. The fuel tanks are more expensive, while the engines require high-pressure vessels for CNG and a highly complex fuel system for LNG with sophisticated tanks, valving and heat exchangers to reduce the fuel from its frozen state to a gaseous one. “I don’t see how manufacturers are going to take those costs out of the equation,” Carrick says.

“You have to run a lot of miles with the trucks if you’re going to make this investment pay off,” says Phil Crofts, director of marketing for Burr Ridge, Ill.-based Dillon Transport, which is testing a Peterbilt with a 12-liter LNG engine. “At first, we were unfamiliar with the new engine, the electronics and things like that,” Crofts says. “But the fuel is performing almost as advertised.”

Crofts says the truck isn’t getting the utilization Dillon had hoped. “If you’re only going to run 100,000 miles a year, natural gas is probably not going to pay off,” he says. “We run our trucks 250,000 to 300,000 miles in a year, and we’re saving some sizable chunks of money with those mileage figures. But a lot of that goes out in increased maintenance and in other areas – spark plugs, oil changes and things like that.”

When asked about the ROI for natural gas, Shefchik looks at the total cost of the vehicle for its lifetime, but he also always gives the same advice. “If you have a fueling station that is on one of your routes, buy a truck or a couple of trucks and test them,” he says. “Why not get started so you have some experience and then you can move as the industry moves?”